Intension of our examination is to investigate changes in ploidy during radiotherapy and to find out radiosensitive cloni of tumors.In November 1993 a 83-year old patient with a carcinoma of the vulva was introduced to the Strahlenklinik der Städtischen Kliniken Offenbach. It was a stage T3N0M0 tumor (UICC 1990). We performed daily smears of the tumor surface during radiotherapy. The smears became air-dried before fixation in PBS/formaldehyde and staining with the Feulgen procedure. The DNA-analyse took place at an image-analysis-system (Ahrens Cytometry Analyse System).Nine and 16 weeks after radiotherapy the patient showed clinically complete remission. Macroscopically and histopathologically there was no tumor found. The ploidy-analysis shows the take off of the hyperpentaploid cells on radiotherapy. Nine and 16 weeks after finished therapy there are no more hyperpentaploid cells detectable.The DNA-determination is a senseful completition to estimate the success of therapy. It is possible to earn informations about the behaviour of different tumor cell populations during the course of radiotherapy. The hyperpentaploid cells are a radiosensitive part of the tumor. For being able to judge about the route of this cells, if they are proliferating cells of a peritriploid tumor population, or if they proliferate themselves, are some more examinations and additional investigations necessary.